1. The Field of the Invention
This filed to the use of ink jet printers to make biological microarrays.
2. The Relevant Technology
Advances in biological and chemical science are demanding the testing of large numbers of samples in parallel. For example, the sequencing of human and animal genomes has created a need to determine the function of genes through expression studies. In this field, in pharmacogenomics and toxicology screening, and in many other applications, there is a need to test a large number of interactions between probe and target.
A technology which has emerged to address this need is the microarray, also known as the DNA microarray or biochip, and by other terminology. This consists of a substrate on which a compact array of biological or chemical samples, known as probes, is immobilised. The microarray is exposed to a sample, known as the target, which is to be tested against the probes. The interactions are recorded by suitable instrumentation and the data is manipulated.
Microarrays are made at present by two methods: the probes can be synthesised on the array, by applying constituents of the probes to build them up in situ; or pre-synthesised probes can be spotted onto the array. This invention relates to the latter method.
The task of spotting a microarray consists of transferring extremely small amounts of many different liquids from separate reservoirs to closely spaced positions on a number of microarrays. There may be anything from tens of different liquids to hundreds of thousands of them, supplied typically in multiple 96, 384 or 1536-well microtitre plates. Some tens or hundreds of substrates need to be spotted with each of the liquids; typical spot volumes are of the order of a nanolitre, and spots may be separated by a few hundred microns.
Spotting is achieved at present in two main ways: in the first method, pins are dipped into the wells to pick up samples of the liquids, and then moved on a three-axis transport to touch the substrates and deposit drops. Several pins may be used in parallel to speed up the spotting.
There are disadvantages to this technology: the pins have to be washed and dried before picking up samples of another set of liquids. The pins have to touch the substrate, which requires high precision, carries a risk of damage, and is slow. The volume of liquid spotted is rather large, is not well controlled and cannot be varied easily. The configuration of spots on the microarray corresponds to the arrangement of liquids in the wells, as the pins are all brought into contact with the substrate simultaneously. A considerable proportion of each liquid is wasted.
The second method of spotting is to project the liquid through the air onto the substrate, without contact. In principle, ink jet printing technology is eminently suitable: it produces small droplets, very reproducibly, and positions them accurately on the substrate. In some cases, the droplets are sufficiently small that multiple droplets can be applied to a given spot to vary its volume. Ink jet printing is very rapid, and is entirely flexible as to what liquid is deposited where on the substrate.
The main difficulty with ink jet technology is that, although some printheads have large numbers of nozzles, they are designed to print typically one or four colours of ink. Their inlets lead to manifolds which connect many chambers, each associated with a nozzle. If such a printhead is applied conventionally to the manufacture of microarrays, the speed of the process is limited by the fact that only one or four liquids is handled at a time, and the fact that there are many nozzles is of little help. The printing itself is very quick, and it is the process of emptying and refilling the printhead which determines the overall manufacturing time.
Other difficulties with ink jet printheads are: some use local boiling of the liquid to eject drops, which could damage some biological samples; others are constructed from materials incompatible with the chemicals to be printed onto microarrays; some are designed for office printers, and are unsuitable for third party integration into industrial systems; and others are designed for industrial use, but require large volumes of liquid to operate.
For the reason given above, standard ink jet printheads are not used in the manufacture of microarrays; rather adapted printheads or devices akin to printheads, are used instead. These do not take advantage of the manufacturing capabilities of ink jet companies, and do not handle large numbers of liquids.